In this contribution we report a study of Cs adsorption on clean and covered with different amounts of Se, Si(1 0 0) 2 X 1 surfaces, by low energy electron diffraction Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements in UHV. The presence of Se on the Si(1 0 0) surface does not affect the sticking coefficient of Cs but it increases the binding energy and the maximum coverage of the subsequently deposited Cs. The Cs overlayer on Se-covered Si(1 0 0) surfaces interacts strongly with the substrate, reduces the Si and forms CsSe and SixCsySez compounds. The strong Cs-(Se/ Si(1 0 0)) interaction leads to a complete disorder of the surface. (C) 2004 Elsevier B.V. All rights reserved.